In chemical experiments, during the process of the external operations, often biochemical substances that are harmful to humans are produced. Fume hoods are used as one type of equipment to prevent these biochemical substances from being diffused into a room and to prevent them from coming into contact with the human body. Typically, fume hoods are provided with an enclosure having a sash that can be opened either vertically or horizontally, where an operator in the laboratory can access the inside of the enclosure through the sash. So that the operator will not be exposed to harmful biochemical substances during the operations using the fume hood, the enclosure is connected to a local exhaust duct that removes the biochemical substances.
The room pressure controlling system is a system for maintaining a constant pressure within a room so that when biochemical substances are handled within a fume hood, the airflow rate of the local exhaust duct is adjusted so as to maintain a specific speed for the planar airflow rate within the sash plane so that there is no backflow of biochemicals into the room, so that biochemical substances do not leak out of the room and that contamination, and like, will not leak into the room (See, for example, Japanese Unexamined Patent Application Publication H9-201540). FIG. 8 is a diagram illustrating the structure of a conventional room pressure controlling system. The room pressure controlling system includes fume hoods 101-1 and 101-2 that are disposed within the room 100; local exhaust ducts 102-1 and 102-2 that are connected to the fume hoods 101-1 and 101-2; a supply air duct 103 for supplying supply air to the room 100; a common exhaust duct 104 for the air of the room 100; local exhaust air valves EXV1 and EXV2 for regulating the airflow rates of the local exhaust ducts 102-1 and 102-2; a supply air valve MAV for regulating the airflow rate of the supply air duct 103; a common exhaust air valve GEX for regulating the airflow rate of the common exhaust duct 104; controllers 105-1 and 105-2 for controlling the local exhaust air valves EXV1 and EXV2; a controller 106 for controlling the supply air valve MAV; a controller 107 for controlling the common exhaust air valve GEX; and communication lines 108 for connecting together the various controllers 105-1, 105-2, 106, and 107. The fume hoods 101-1 and 101-2 are provided with sashes 111-1 and 111-2 that can be opened and closed, and sash sensors 112-1 and 112-2 for detecting the degrees of opening of the sashes 111-1 and 111-2.
In this type of room pressure controlling system, in order to maintain the pressure of the room 100 at the setting value, the degrees of opening of the supply air valve MAV and the common exhaust air valve GEX, and of the local exhaust air valves EXV1 and EXV2, are controlled so that the supply airflow rate of the supply air duct 103, the exhaust airflow rate of the common exhaust duct 104, and the local exhaust airflow rates of the local exhaust ducts 102-1 and 102-2 satisfy the relationship of “Supply airflow rate=common exhaust airflow rate+local exhaust airflow rates+offset airflow rate.” Moreover, in recent years a PCV (pressure control valve) function, for performing stabilized pressure control by adjusting the degree of opening of the PCV valve based on a pressure differential by measuring the pressure differential between the inside and the outside of the room has been added as well. This PCV function is achieved through causing the room pressure controlling operations to be performed as well in addition to the actual functions of either the supply air valve MAV or the common exhaust air valve GEX.
Additionally, there has been a proposal for a system that reduces the supply airflow rate and the common exhaust airflow rate, while maintaining a constant pressure differential between the inside of the room and the outside of the room, in order to achieve energy conservation through reducing the amount of air exchanged, because it is possible to reduce the amount of air exchange during time bands wherein people are not present, such as at night and on weekends when no operations are being performed (See, for example, Japanese Unexamined Patent Application Publication 2004-138270 (“JP '270”)). Even when the airflow rates are varied in this way, it is still necessary to control the airflow rates so that the pressure differential between the inside and outside of the room will always be the same.
The process for varying the airflow rates disclosed in JP '270 is performed every day on weekdays. In an example when switching the time band from daytime to nighttime, the degree of opening of the PCV (which is either the supply air valve or the common exhaust air valve) is adjusted so as to maintain always a constant pressure differential between the inside and the outside as the supply airflow rate and the common exhaust airflow rate are both reduced gradually. Moreover, in an example of switching the time band from nighttime daytime, the degree of opening of the PCV is corrected so that the pressure differential between the inside and the outside of the room is always constant while the supply airflow rate and the common exhaust airflow rate are both increased gradually. In operations that perform fine adjustments to the opening of the PCV in this way, if, for example, the PCV is opened too much, then, as the correcting operation, an operation is performed in the direction of closing the PCV, and thus a reversing operation is produced in the PCV. Because the life expectancy of the valve is greatly influenced by the frequency of these reversals, there is a problem in that the life expectancy of the valve is shortened through frequently producing reversing operations in the valve. When a valve that handles a PCV function fails, it tends to cause large failures in the room pressure control.
The present invention was created in order to solve the problems set forth above, and the object thereof is to provide a room pressure controlling system able to extend the service life of a valve that is provided with a PCV function and to be able to extend the up-time, and the time between maintenance.